Fair enough, I don’t drink many sweet drinks and tried to carbonate soda water that way.
Re head space, you also need to make sure you’re leaving enough when you open it. You’re only very lightly carbonating with 3.5 g. Most soda water and softdrinks have a head pressure of around 70 psi, the bottles can actually spike to around 150 when dropped which is sort of terrifying. Without the addition of all the crap they add to slow nucleation bottles can erupt very merily when opened!
When making my own soda water (I just hook bottles up to a regulated stream of co2 and shake) I do actually add some but only about 1/8 of a teaspoon per litre.
Making things slightly salty makes them nice and I find it helps reduce some of the harsh acid taste the carboxylic acid. Also it seems to modify how the bubbles form which is interesting. Maybe a surface tension of water effect? I’m unsure there.